The hearts of bivalved molluscs are structurally and functionally analogous to those of vertebrates, and are favorable model systems for studying the general mechanisms underlying rhythmical activity in cardiac muscle. This investigation of molluscan hearts will focus on two agents which profoundly modify rhythmicity: Peak C and 5- hydroxytryptamine (5HT). Peak C is found ubiquitously in molluscan central nervous tissue and is probably a small polypeptide. It is a cardio-excitor agent, inducing spontaneous activity in quiescent molluscan hearts, while regularizing or augmenting the beat of active ones. Alhough Peak C has been purified, its composition is still unknown; the structure will be determined by N-terminal degradation, and proved by synthesis. The purified native Peak C now available will be used to study the pharmacology of this substance on various molluscan muscles, but especially hearts; electrical and mechanical responses will be recorded. Finally, the subcellular distribution of Peak C in molluscan ganglia will be determined by differential centrifugation. Since, in contrast to peak C, the effects of 5HT on molluscan hearts is variable, the actions of this drug will be compared on the hearts of 4 animals: Mercenaria mercenaria, Mytilus edulis, and two subspecies of Modiolus demissus. These mycordia vary in their responses to 5HT, and in the ionic dependence of their spontaneous excitation. This investigation will focus on the calcium compartments involved in 5HT action and will proceed along three lines: Organ bath experiments will be carried out to determine the effects of lanthanum, caffeine and low calcium on the 5HT modification of rhythmicity. The effects of these same agents on 45 Ca ions distribution and movement will be determined in uptake and efflux experiments. Finally, an attempt will be made to identify the morphological sites of calcium acumulation by the aplication of electron microscopical techniques.